A glass has hitherto been widely used as a substrate for a flat panel display, such as a liquid crystal display, a hard disk, a filter, a sensor, or the like. In recent years, in addition to a conventional liquid crystal display, an OLED display has been actively developed by virtue of its self-luminescence, high color reproducibility, wide viewing angle, high-speed response, high definition, and the like. Some of the developed OLED displays have already been put to practical use. Meanwhile, a liquid crystal display or an OLED display for a mobile device, such as a smartphone, requires a screen with ultra-high definition because the display has a small area but needs to display a lot of information. The display also requires high-speed response because a moving image is displayed thereon.
In such application, an OLED display or a liquid crystal display driven by a LIPS is suitable. The OLED display emits light when a current flows through an OLED element serving as a constituent of a pixel. Therefore, a material exhibiting low resistance and a high electron mobility is used as a drive TFT element. As such material, an oxide TFT formed typically of indium gallium zinc oxide (IGZO) has attracted attention, aside from the above-mentioned LIPS. The oxide TFT has low resistance, a high mobility, and can be formed at relatively low temperature. The conventional p-Si⋅TFT, in particular the LIPS, is liable to have variations in TFT characteristics upon its formation on a large-size glass substrate, owing to instability of an excimer laser to be used in polycrystallization of an amorphous Si (a-Si) film. Therefore, in a TV application or the like, display unevenness is liable to occur in a screen. In contrast, the oxide TFT is excellent in homogeneity of TFT characteristics upon its formation on a large-size glass substrate. Therefore, the oxide TFT has attracted attention as a potential TFT formation material, and some of the oxide TFTs have already been put to practical use.
A glass substrate to be used for a high-definition display is required to have various characteristics. In particular, the glass substrate is required to have the following characteristics (1) to (4).
(1) To contain an alkaline component (in particular, a Li component or a Na component) at a low content or be substantially free of the alkaline component, because an alkali ion is diffused during heat treatment into a semiconductor substance having been formed into a film and causes degradation in the characteristics of the film when the glass substrate contains the alkaline component at a high content.
(2) To undergo less thermal shrinkage, in particular have a high strain point, because the glass substrate is subjected to heat treatment at several hundred degrees C. in steps of film formation, annealing, and the like, and hence a pattern shift or the like is liable to occur when the glass substrate undergoes thermal shrinkage during the heat treatment.
(3) To have a thermal expansion coefficient close to that of a film member to be formed on the glass substrate (for example, of a-Si or p-Si), for example, have a thermal expansion coefficient of from 30×10−7/° C. to 45×10−7/° C. A thermal expansion coefficient of 40×10−7/° C. or less improves thermal shock resistance as well.
(4) To have a high Young's modulus (or a high specific Young's modulus) in order to suppress failures attributed to the deflection of the glass substrate.
The glass substrate is also required to have the following characteristics (5) and (6) from the viewpoint of manufacturing the glass substrate.
(5) To have excellent meltability in order to prevent melt defects, such as bubbles, stones, and cords.
(6) To have excellent denitrification resistance in order to prevent devitrified foreign matter from being mixed in.